The general field is a Kalman filter and more specifically an application of Kalman filtering algorithm in an OFDM based communication system.
Orthogonal Frequency Division Multiplexing OFDM has been widely applied in wireless communication systems such as DVB-T/H, 802.11x wireless LAN and 802.16 wireless MAN due to its high bandwidth efficiency and robustness to multipath fading. Due to the fact that the wideband wireless channel is frequency selective and time varying, channel estimation must be performed continuously and the received OFDM subcarriers must be corrected by the estimated CTFs. FIG. 1 shows a generic OFDM receiver.
In DVB systems, channel estimation is performed by inserting known scattered pilots at predefined subcarrier locations in each OFDM symbol (see ETSI EN 300 744 V.1.4.1 “Digital Video Broadcasting (DVB): Framing Structures, channel coding, and modulation for digital terrestrial television”), normally referred to as “comb-type” pilot channel estimation.
FIG. 2 shows the scattered pilots insertion in DVB-T transmitters. The PPS pilots and the continual pilots are not shown for sake of clarity. The comb-type channel estimation consists of algorithms to first estimate the channel transfer functions at the pilot locations and then to interpolate the channel transfer function in time and frequency domain to get the channel estimates for all the OFDM subcarrier locations.
As shown in FIG. 2, in comb-type pilot based channel estimation the Ns scattered pilots are inserted uniformly into the OFDM spectrum according to the following rules:
For the symbol of index l (ranging from 0 to 67), carriers for which index k belongs to the subset {k=Kmin+3×(l mod 4)+12p|p=int, p≧0, k∈[Kmin; Kmax]} are scattered pilots.
Where p is an integer that takes all possible values greater than or equal to zero, provided that the resulting value for k does not exceed the valid range [Kmin; Kmax].
Assume that for current symbol of index l, the Ns inserted scattered pilots according to the above rule are: Xs, s=0, 1, . . . Ns−1, the corresponding received subcarriers at the scattered pilot locations are: Ys, s=0, 1, . . . Ns−1, the channel frequency response at the pilot subcarrier locations can be represented as: Hs, s=0, 1, . . . Ns−1, then the Least-Square estimate of the channel frequency response at the pilot subcarrier locations is given by:
                    H        ^            s        =                  Y        s                    X        s              ,      s    =    0    ,  1  ,      …    ⁢                  ⁢    N    ,      -    1  
The above LS estimation is sensitive to noise and ICI, MMSE estimation is known to provide better performance than LS estimation. However, MMSE estimation requires matrix inversion at each iteration, thus not practical for implementation.
In this disclosure, a simplified Kalman filter is provided which reduces the noise effects of the LS estimation. Simulation shows that the simplified Kalman filter is very effective in removing the noise effects, and the overall system performance will be improved by up to 2 dB.
A scalar Kalman filter is applied for a Least-Square estimated value Hs at s. The filter has an input for receiving Hs, a filter equation and an output for the corrected estimated value Hsk for the kth variable. The filter equation is Hsk=KgainSn[k] wherein: correction Sn[k]=S+Kn(Hs−S); prediction of the correction S=KaSn[k]; Kalman filter gain Kn=P/(1+P); minimum predication MSE P=Ka2Pn[k]+Kb; minimum MSE Pn[k]=P(1−Kn); and Ka, Kgain and Kb are constants.
A receiver includes an OFDM demodulator, a channel corrector and a channel estimator; and wherein the channel estimator is a Least-Square estimator of a channel frequency response Hs of a subcarrier k. The channel estimator includes the simplified Kalman filter. The constants Kgain, Ka and Kb may be selected as a function of the modulation mode of the subcarrier. The channel estimator processes scattered pilots whose locations s repeats its pattern every r symbols; and the channel estimator includes r*Ns Kalman filters.
The filter gain Kn may also be a constant selected as a function of the modulation mode of the subcarrier. The filter equation is performed in software.
These and other aspects of the present disclosure will become apparent from the following detailed description of the disclosure, when considered in conjunction with accompanying drawings.